The oil sands in Northern Alberta constitute one of the largest hydrocarbon reserves in the world. Oil sands are a combination of bitumen, quartz sand, clay, water and trace minerals. Bitumen can be recovered from oil sands using two main methods: open-pit mining and in situ drilling. Approximately 20% of the oil sands lie close enough to the earth's surface to be mined.
The key characteristic of Alberta oil sand that makes bitumen economically recoverable is that the sand is hydrophilic and encapsulated by a water film. The presence of this water film prevents the direct contact of bitumen and sand. Thus, by slurrying mined oil sand with water, the bitumen is allowed to liberate from the sand grains and move to the aqueous phase. In a water-based bitumen extraction process, the efficiency of bitumen separation is determined by the following four fundamental steps: bitumen liberation, bitumen droplets coalescence, bitumen droplets aeration, and bitumen flotation.
There is a strong correlation between bitumen droplet size and bitumen recovery. In general, good bitumen flotation in a primary separation vessel or PSV and, hence, good bitumen recovery, can be obtained if the average size of bitumen droplets is greater than 400 μm. On the other hand, the recovery can be very poor if the average size of bitumen droplets is smaller than 200 μm. For small bitumen droplets (e.g., ≦200 μm), bitumen flotation can be challenging. In addition, the density difference between bitumen and water is very small under normal extraction conditions. Bitumen droplets must be aerated to become lighter than water so that they are able to float to the top of a separation vessel to be recovered. Both bitumen droplets coalescence and aeration are critical for bitumen extraction.
It is generally believed that when a bitumen droplet is very small, its probability of collision with another droplet for coalescence and with a bubble (e.g., air bubble) for aeration is very low, This is because a small droplet does not possess sufficient kinetic energy to deviate from the streamlines and to displace the intervening liquid layer to collide with other bitumen droplets and/or bubbles. In addition, the interaction forces between bitumen droplets and between bitumen droplets and bubbles are naturally repulsive due to the operation conditions normally used (e.g., slurry pH in the range of 7.5 to 9.5). The presence of strong repulsive forces makes coalescence and aeration difficult. Small bitumen droplets are also difficult to rise to the top of a separation vessel to be recovered.
Several different water-based bitumen extraction processes have been developed throughout the years. One such extraction process is commonly referred to in the industry as the “hot water process”. In general terms, the hot water process involves feeding the mined oil sand into a rotating tumbler where it is mixed for a prescribed retention time (generally in the range of 2 to 4 minutes) with hot water (approximately 80-90° C.), steam, caustic (e.g., sodium hydroxide) and naturally entrained air to yield a slurry that has a temperature typically around 80° C. The bitumen matrix is heated and becomes less viscous. Chunks of oil sand are ablated or disintegrated. The released sand grains and separated bitumen flecks are dispersed in the water. To some extent bitumen flecks coalesce and grow in size. They may contact air bubbles and coat them to become aerated bitumen. The term used to describe this overall process is “conditioning”. The conditioned oil sand slurry is then subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
More recently, a number of bitumen extraction processes have been developed which rely on conditioning of the oil sand slurry in a hydrotransport pipeline. Pipeline conditioning or hydrotransport is disclosed in Canadian Patent No. 2,029,795 and U.S. Pat. No. 5,039,227. In one such extraction processes, heated water (typically at 95° C.) is mixed with the dry as-mined oil sand at the mine site in predetermined portions using a device known as a “cyclofeeder”, to form an aerated slurry having a temperature in the range of 40-70° C., preferably about 50° C. The slurry is then pumped to the extraction plant through several kilometres of pipeline, where conditioning (i.e., lump digestion, bitumen liberation, bitumen droplet coalescence and bitumen droplet aeration) occurs. The conditioned oil sand slurry is then subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
A low energy extraction process for extracting bitumen from oil sand is disclosed in Canadian Patent No. 2,217,623 and U.S. Pat. No. 6,007,708. This process involves mixing the mined oil sand with water in predetermined proportions in a mix box located near the mine site to produce a slurry containing entrained air and having a controlled density in the range of 1.4 to 1.65 g/cc and preferably a temperature in the range 20-40° C. The slurry is then pumped through a pipeline to condition the slurry. Once again, the conditioned oil sand slurry is subjected to gravity separation, generally in a PSV, to produce a bitumen froth product.
For good processing ores, the bitumen droplets generated during conditioning of oil sand slurry are normally big enough to obtain desirable bitumen recovery. However, for problem ores (e.g., low-grade high-fines ores), the average size of the bitumen droplets generated are often small, leading to poor extraction performance. In order to improve the overall performance of a water-based bitumen extraction process, especially for problem ores, the efficiency of bitumen droplets coalescence and aeration needs to be improved to increase the average size of bitumen droplets.
In many existing water-based bitumen extraction processes, caustic (e.g., sodium hydroxide) is used as a process aid to improve the overall performance. Caustic helps to release natural surfactants and affects surface properties of bitumen, sand, and clays. The use of caustic reduces the attachment of fine solid particles on bitumen surface (so called slime coating), thus facilitating the coalescence and aeration of bitumen droplets. However, the use of caustic increases the slurry pH and thus increases the repulsions between bitumen droplets and between bitumen droplets and bubbles. The use of a conditioning step in existing extraction processes with mechanical energy input also helps the coalescence and aeration of bitumen droplets, improving the overall performance.
However, when processing problem ores, the use of caustic and the existing conditioning step often does not provide enough improvement to bitumen droplet coalescence and aeration to obtain desirable performance. In a hydrotransport pipeline, bitumen droplets and bubbles travel in the same direction. As a result, small bitumen droplets may not possess sufficient kinetic energy to deviate from the streamlines and to displace the intervening liquid layer to collide with other bitumen droplets and/or bubbles. Also, as the bitumen droplets and bubbles essentially travel in parallel, the probability for them to collide to each other is low. Thus, it is believed that the efficiency of bitumen droplets coalescence and aeration in such a pipeline may be limited.
Therefore, there is a need in the industry to improve the efficiency of bitumen droplets coalescence and aeration during oil sand extraction, especially for poor processing ore.